1. Field of the Invention
This invention relates generally to variable frequency oscillators and more particularly is directed to variable frequency oscillators having a closed control loop by which the oscillator is locked to a predetermined frequency.
2. Description of the Prior Art
As an oscillator having a stabilized frequency, it has been previously proposed to employ a locked oscillator or frequency synthesizer utilizing a phase-lock-loop which comprises a reference oscillator, a variable frequency, usually voltage-controlled oscillator, a frequency divider or multiplier for changing the frequency of the output of the variable frequency oscillator to substantially the same frequency as the output of the reference oscillator, and a phase comparator for comparing the phase of the output of the reference oscillator with the phase of the frequency-change output of the variable frequency oscillator and controlling the variable frequency oscillator in response to the compared output. When the frequency divider or multiplier is fixed, the frequency of the variable frequency oscillator is stably locked to a predetermined frequency, and a signal having that desired frequency can be reliably obtained.
In such prior art locked oscillator or frequency synthesizer, it is possible to change the frequency to which the variable frequency oscillator is to be locked by changing the dividing rate or multiplying rate of the frequency divider or multiplier. However, if the desired frequency to which the variable frequency oscillator is to be locked is relatively high as compared with the output frequency of the reference oscillator, so that the output frequency of the variable frequency oscillator must be divided by a great number so as to substantially correspond to the frequency of the output of the reference oscillator, the response time of the frequency divider becomes undesirably long, and the dividing rate of the frequency divider must be large.
The long response time of the frequency divider causes instability in the frequency locking of the variable frequency oscillator, and the large frequency dividing rate requires a complicated and expensive frequency divider.